Renewable sources of insulin-producing beta-cells for transplantation therapy of diabetes mellitus will most likely derive from the managed development of surrogate cells, such as embryonic or adult 'stem' cells. This goal would be facilitated greatly by a thorough understanding of the extrinsic morphogens and cell-autonomous factors that control embryonic pancreatic development. P48/PTF1A is a pancreas-restricted bHLH transcription factor originally thought to function selectively in specifying acinar cell fate. Recently P48 has been shown to be necessary for the formation of islet as well as acinar tissue. The functional form of P48 in pancreatic acinar cells is the heterotrimeric PTF1 complex. We have discovered that PTF1 from adult pancreas comprises P48, one of the common E proteins (E12, E47 or HEB), and SUH-L (aka RBPSUHL), a mammalian homologue of Drosophila Su(H). We have shown that the mammalian orthologue of Su(H) (SUH, aka RBPSUH) can also form a trimeric complex with P48 and an E protein and that this ability is conserved for the Drosophila proteins. We propose that PTF1 containing either SUH or SUH-L and in concert with Notch-signaling may control cell-fate decisions during pancreatic organogenesis. We will test this notion through three Specific Aims. Aim 1 will use a genetic approach for depleting P48 at different developmental stages in utero to determine which processes require P48 and to identify direct P48-target genes. Aims 2 and 3 will determine which of the P48-dependent processes also require SUH or SUH-L and whether that requirement includes their participation in a PTF1 complex, as predicted.